Large quantities of tar sands can be found in relatively shallow deposits in various localities throughout the world. The largest of these deposits is found in the Canadian province of Alberta. These deposits which are commonly referred to as Athabasca tar sands contain upwards of 350 billion barrels of recoverable bitumen and underlie more than 3000 square miles at depths of 0 to 2000 feet. A large part of these tar sands can be recovered by open pit strip mining. The province of Alberta is well known for its long harsh bitter cold winters and its short but often hot summers. Temperatures over a twelve-month period ranging from a low of -50.degree.F to a high of 95.degree.F are not unusual in northern Alberta. Mining tar sands under these conditions presents some unique problems of digging and transportation heretofore not encountered.
Bituminous tar sands such as the Athabasca tar sands comprise a siliceous material, generally having a size greater than that passing a 325 mesh screen, saturated with a relatively heavy, viscous bitumen in quantities of from 5 to 21 weight percent of the total composition. More typically, the bitumen content of the sands is about 8 to 15 percent. This bitumen is quite viscous and contains typically 4.5 percent sulfur and 38 percent aromatics. Its specific gravity at 60.degree.F ranges typically from about 1.00 to about 1.06. The tar sands also contain clay and silt in quantities of from 1 to 50 weight percent of the total composition. Silt is normally defined as mineral which will pass a 325 mesh screen but which is larger than 2 microns. Clay is mineral smaller than 2 microns including some siliceous material of that size.
There are several well-known processes for effecting separation of bitumen from the tar sands. In the so-called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of a neutral salt in water. The combined mass is then subjected to a pressure or gravity separation.
In the hot water method, the bituminous sands are jetted with steam and mulled with a minor amount of hot water at temperatures in the range of 140.degree.F to 210.degree.F. The resulting pulp is dropped into a stream of circulating hot water and carried to a separation cell maintained at a temperature of about 150.degree.F to 200.degree.F and usually 185.degree.F. In the separation cell, sand settles to the bottom as tailings and bitumen rises to the top in the form of an oil froth. An aqueous middlings layer containing some mineral and bitumen is formed between these layers. A scavenger step may be conducted on the middlings layer from the primary separation step to recover additional amounts of bitumen therefrom. This step usually comprises aerating the middlings as taught by K. A. Clark, "The Hot Water Washing Method", Canadian Oil and Gas Industries 3, 46 (1950). These froths can be combined, diluted with naphtha, and centrifuged to remove more water and residual mineral. The naphtha is then distilled off and the bitumen is coked to a high quality crude suitable for further processing. The hot water process is described in detail by Floyd et al. in Canadian Pat. No. 841,581 issued May 12, 1970.
The tar sands must be mined from their deposits for charge into the particular process used to effect separation of the bitumen from the sands. In one particular mining operation, overburden is first removed from he deposits. This involves the stripping away of trees, muskeg, and earth which overlie the tar sands. The sands are then mined by giant bucketwheel excavators. The digging wheels on such excavators can have ten or more buckets each bucket capable of biting out and holding nearly two tons of sand. Sand dug by the wheel travels the length of the digger by conveyor to the discharge boom and is dropped on the first of several conveyor belts.
One particular operation utilizes a first conveyor which is 4,600 feet long, 60 inches wide, and made of one-inch thick rubber which is a vulcanized mixture of about 15 percent natural rubber and about 85 percent styrene-butadiene copolymer rubber on steel cords. It travels at 1,050 feet per minute, or about 12 miles per hour. The sand is dumped from the first conveyor onto a second which is 60 inches wide and about 1000 feet long. As diggers get farther from the initial mining area, the second conveyor is extended and ultimately both conveyors stretch several miles. The second conveyor drops sand onto a third conveyor. This third belt, 72 inches wide, runs 1,350 feet to the separation plant. This system has been used commercially at Fort McMurray, Alberta, for a number of years.
Although belt conveyors have been widely used in other mining operations, there are some new problems peculiar to the handling of tar sands. Up until now conveyor belts were made from natural rubber or styrene-butadiene copolymer or mixtures of the two. These compounds are selected because they retain good flexibility over a wide temperature range, e.g., -60.degree.F to 150.degree.F. However, when these belts are used in a tar sands mining operation, it has been observed that large quantities of the sands stick to the belts causing fouling and a decrease in belt capacity.
Depending on the character of the feed material and the outside temperature, the layer of sticking sands attains a thickness of as much as one-half inch. In warm weather the belt deposit increases in toughness and thickness. In cold weather, the deposit becomes frozen in transit from the mining area to the processing area. Steam jets and scrapers have been proposed for the purpose of removing the deposit but none of these means has been fully satisfactory. Accumulation of sands on the conveyor belts causes unbalanced loads, straining problems, and additional wear on pulleys, idlers, and scrapers.
Canadian Pat. No. 922,655 issued Mar. 13, 1973 to William Hogg discloses an improvement to a process for transporting bituminous tar sands on a conveyor belt between a receiving area and a discharge area which generally comprises applying a layer of an aqueous liquid medium to the surface of the conveyor prior to transporting tar sands on the belt surface. The aqueous layer reduces the tendency of the tar sands to stick to the belt when discharged at the discharge area. When operating this process at temperatures above freezing, it has been found to be reasonably effective. However, when the temperature falls below freezing, it is necessary to add freezing point depressants such as alcohols to keep the water from freezing on the belt surface. Ethylene glycol is one of the preferred additives because of the safety of its use as well as its compatibility with subsequent tar sand processing steps. However, the relatively high expense of ethylene glycol, particularly in colder climates, reduces its attractiveness as an answer to the conveyor belt cleaning problem. Thus, although the disclosure of the above Canadian patent provides one technically feasible means of reducing the problems of transporting tar sands on a conveyor belt system, substantial room for improvement still exists.
One method employed to overcome the problem of tar sands sticking to the surface of natural rubber or styrenebutadiene rubber conveyor belts has been the addition of kerosene to the surface of the belt before the tar sands are placed thereon. During cold weather operation, that is at ambient temperatures below 10.degree.F, this method of keeping conveyor belts reasonably clean has been generally acceptable. However, when the temperature rises above 10.degree.F the natural rubber or styrene-butadiene rubber will absorb the aromatic hydrocarbon liquids of the kerosene and begin to swell. The aromatic swollen belt would often expand to the degree that it becomes inoperable and the belt must be shut down.
One approach to overcome the swelling of the conveyor belt caused by application of liquid hydrocarbons is to select a rubber composition for the belt surface which is resistant to swelling. A rubber well-known for its resistance to petroleum liquids is nitrile rubber.
In The Vanderbilt Rubber Handbook edited by G. G. Winspear (R. T. Vanderbilt, Inc., New York, 1968) pages 99 to 118, methods of preparation as well as physical and chemical characteristics of nitrile rubber are disclosed. It is disclosed in this text that the oil and chemical resistance of nitrile rubber is the major factor dictating its use. The oil resistance of compounds based on nitrile rubber is determined by the acrylonitrile content of the nitrile rubber. Also, it is disclosed that the low temperature properties of nitrile rubber compounds vary with the acrylonitrile content of the polymer and the type of plasticizer incorporated therein.
It is also disclosed that nitrile rubber has a wide range of compatibility with other polymers, making possible a great variety of unique and desirable properties. Blends can be made with vinyl chloride, phenolic and ABS resins, SBR, polychloroprene, cis polybutadiene, chlorosulfated polyethylene, thiokol, and to a certain extent, natural rubber. All of these blends are used to impart variations in processing, ozone resistance, low temperature flexibility and cost. The use of nitrile rubber for conveyor belts is disclosed. Yet with all this information available, the industry has up until this point been unable to provide a flexible rubber conveyor belt suitable to transport tar sands in an open pit mine without having to shut the belt down because of tar sands sticking to the belt surface.
By the method of the present invention using the novel elastomer compositions herein provided, tar sands can be efficiently and economically transported in an open pit mine under a wide range of weather and temperature conditions.